1. Field Of The Invention
The present invention relates to a method for retaining a passive post and reinforcing endodontically treated teeth. The invention also relates to a specially designed vacuum apparatus for use in the method of retaining a passive post and reinforcing endodontically treated teeth. The invention further relates to a specially designed passive post for use in the method of retaining a passive post and reinforcing endodontically treated teeth.
2. Description Of The Related Art
The basic principals of root canal treatment are debridement, sterilization and obturation of the root canal. If these principals are successfully met, a positive prognosis is predicted for endodontically treated teeth.
However, there are situations when a successful root canal therapy is not enough to save an endodontically treated tooth from extraction. These situations are directly related to post-endodontic restoration, and, more specifically, when there is a need for intracoronal restoration.
Post designs, luting cements and status of remaining dentin have a great influence on the success of such a restoration. However, known post designs are made to fulfill the retentive needs of the coronal restoration and ignore the important need of the abutment root for reinforcement against fracture. This is especially true when the tooth has been weakened by caries, root canal treatment and post preparation.
Many factors have been documented that cause the root to split or fracture. Some of these include: dehydration of pulpless teeth, reduction of dentinal structure, reduced elasticity of dentin, stress from post design and cementation, corrosion of the metal post and residual stresses from lateral condensation of gutta percha.
One post design which has been found to have capabilities for protecting pulpless teeth against root fracture by distributing stresses evenly on the body of the root is the parallel-sided serrated post design.
Felton, D.A., Webb, E.L., Kanoy B.E., and Dugnoni J. (Threaded endodontic dowels: effect of post design or incidence of root fracture, (1991)) reported that the parallel-sided serrated post design's main disadvantage is the low retentive capabilities compared to most other post designs. Even with such design, some endodontically treated teeth may still carry the potential for root fracture, such as in the case of a large pulpal canal in immature teeth with little remaining dentin. Posting such teeth may lead to definite root split and eventually extraction.
Luting cements are used to fill the gaps between the post and surrounding channel walls. They play an important role in post retention and the distribution of stresses transferred through the post to the surrounding root structure.
No one type of cement has been found to have the overall ideal properties needed for cementing endodontic posts. One major problem of luting cements is their poor adhesion to the metal posts and to the surrounding tooth surface.
Removing the smeared layer which covers the channel walls and adding serration or irregularities to the post surface were found to be effective methods to increase the bonding between the luting cement and the post and the surrounding channel walls.
Silicoating the metal surface is a method that has been shown to be effective in bonding any metal surface to resin cement through an intermediate silane layer. This method is used in dentistry for bonding resin to the facial surface in crown and bridges and to cement Maryland bridges and metal rests to the tooth surface.
When an endodontically treated tooth requires a post restoration for the purpose of establishing a solid foundation for a permanent restoration, such as a crown, a post channel is first generated from the top access of the root canal deep into the middle third of the root canal, about 7 mm. This post channel is then occupied by a metal post cemented with a luting agent, such as Zincphosphate, Glass Ionomer or Composite Cement.
The commonly used luting cements have several disadvantages, such as poor bonding to the tooth surface surrounding the post channel and poor bonding to the metal surface of the post. The commonly used luting cements also have problems associated with solubility in water and oral fluids, and erosion and disintegration after long contact with oral fluids.
U.S. Pat. No. 4,936,775, discloses a luting cement comprising nexamethylene diisocyantate adduct of bis-GMA as a binder resin and triethylene glycol dimethylacrylate (TEGDMA) as the dilutent monomer.
The commonly used metal posts also have several disadvantages, such as corrosion which may lead to tooth fracture by the accumulation of corrosion products exerting pressure on the channel walls.
Known passive posts that fit loosely in the post channel and that are retained only from the surrounding luting cement have a low retention value, in the range of 70 to 80 pounds, which results in the potential for dislodgement.
Known active posts that have threads that engage the channel walls by being threaded or screwed in have high retention values, 170 to 190 pounds, however, these posts also exert pressure and stress on the root that may lead to root fracture.
There are two main reasons for using posts and cores: 1) to guarantee an appropriate foundation for the final restoration and 2) to distribute stresses from occlusal forces throughout the remaining tooth structure for the purpose of preventing tooth fracture, Caputo A.A., and Standlee J.P. (Pins and posts-why, when and how. Dental Clinic of North America, 20 299, (1976)).
According to Healey H.J. (Endodontics. St. Louis, The C. V. Mosby Co. pp 267-268 (1960)), the remaining coronal portion of the endodontically treated tooth is more brittle than when it contained vital pulp. Helfer A.R., Melnick S., and Schilder H. (Determination of the moisture content of vital and pulpless teeth. Oral Surgery, 34 (4), 661-669 (1972)) demonstrated that endodontically treated teeth contain 9% less moisture in the calcified tissues than do vital teeth. They concluded that once the moisture has been lost from calcified tissues, it can not be recovered.
Tidmarsh B.G. (Restoration of Endodontically Treated Posterior Teeth. Journal. of Endodontics, 2 (12), 374-375, (1976)) compared a dehydrated pulpless tooth to a dry, dehydrated branch of a tree which "breaks easier than its living counter-part" (pp 374). Tidmarsh also states that an access opening as a part of endodontic treatment removes a very substantial part of the coronal dentin. Consequently, low loads may cause root fracture upon post insertion and may be related to the reduced thickness of dentin.
The use of radioactive phosphorus has demonstrated that the metabolic process in the pulpless teeth decreases rapidly in the coronal dentin. The study by Boyle P.E. (Kronfeld's Histopathology of the Teeth, ed. 4, Philadelphia, Lea & Febiger, (1955)), demonstrated that, as a result of pulp removal, there is a loss of elasticity as compared to the radicular dentin. As a rule, the root portion is less affected if there is normal functioning periodontal support. This may be one of the reasons that the cervical region of the root is more likely to fracture in endodontically treated teeth. Trabert, K.C., Caputo, A.A., and Abou-Rass M. (Tooth fracture, a comparison of endodontic and restorative treatment. Journal of Endodontics, 341-345, (1978)) and Guzy G.E., and Nicholls J.I. (An in vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. Journal of Prosthetic Dentistry, 42 (1), 39-44,(1979)) demonstrated this phenomenon in two separate studies.
One theory has suggested that the intact tooth with no cavity preparation behaves as a prestressed laminate, which withstands higher loads in an unstressed state. When the cavity is generated, it releases the stresses and destroys the prestressed state, leaving the tooth less resistant to fracture under elevated loads, Malcom P. (Cast restoration and cusp flexibility. Thesis of Otago Dunegin, New Zealand, (1973)).
On the question of whether or not endodontically treated teeth are less resistant to fracture, clinicians and researchers have been divided into two groups. One group suggests that the use of endodontic posts enhances the strength of the tooth; the other group believes that such treatment causes a reduction in strength.
Baraban, D.J. (The restoration of pulpless teeth. Dental Clinics of North America, 633-635, (1967)) recommended the use of a cast post as a reinforcement method for pulpless teeth. Perrell, M.L. and Murrof F.I. (Clinical criteria for post and cores. Journal of Prosthetic Dentistry, 28 405-411, (1972)) suggested that once a root canal has been performed on an anterior tooth, it must be reinforced with a post and core.
In a comparative study of restorative techniques for pulpless teeth, Kantor, M.E. and Pines, M.S. (A comparative study of restorative techniques for pulpless teeth. Journal of Prosthetic Dentistry, 38 (4), 405-412 (1977)) demonstrated that the use of prefabricated stainless steel posts, such as Parapost, actually reinforced the pulpless tooth and, in fact, doubled its resistance to fracture as compared to a pulpless tooth without posts. However, when they tested pulpless teeth restored with tapered gold cast posts, they observed that they were less resistant to fracture than the other two groups.
Another study was conducted by Leary, J.M. and Aquilino, S.A. (An evaluation of post length within the elastic limits of dentin. Journal of Prosthetic Dentistry, 57, (3), 277-281, (1987)) in which they applied a compression load at 90 degrees from the CEJ to the long axis of various experimental pulpless teeth groups. Their results indicated that if a tooth structure was removed from the tooth, it became weaker, and that teeth restored with post do show more reinforcement than non-posted teeth. Taylor, A.G. (Dowel abutment crown. Royal Canadian Dental Corps Quarterly, 4, 1-4 (1963)), Johnson, J.K., and Sakamura, J.S., (Dowel form and tensile force. Journal of Prostetic Dentistry, 40, 645-649 (1978)), Barum (1979); Waliszewski, K.J. and Sabola, C.L. (Combined endodontic and restorative treatment considerations. Journal of Prosthetic Dentistry, 40, 152-156 (1978)), Sapone, J., and Lorencki, S.F. (An endodontic-prosthodontic approach to internal tooth reinforcement. Journal of Prosthetic Dentistry, 45, 164-174 (1981)) and others also supported the idea of reinforcing pulpless teeth by using an endo-post.
Guzy and Nicholls, et al. (1979) compared breaking loads of endodontically treated teeth with and without posts cemented using zinc phosphate cement. The load was applied to the clinical crown at 130 degrees. Their results demonstrated that no statistically significant reinforcement was achieved by cementing Kerr endopost no. 100.degree. into a sound endodontically treated tooth. Furthermore, a clinical study by Sorenson, J.A. and Martinoff, J.T. (Intracoronal reinforcement and coronal coverage: A study of endodontically treated teeth. Journal of Prosthetic Dentistry, 51 (6), 780-784, (1984)) generated results that showed no significant increase in resistance to fracture was gained with a post used as a retainer for crown restoration.
In another study, Trabet et al. (1978) used an impact tester to demonstrate that there was no difference in employed fracture resistance between teeth which had not received root canal treatment and endodontically treated teeth without posts. However, when teeth were restored with a stainless steel parapost, size 1.25 mm in diameter, an increased resistance to fracture was obtained. This resistance was not evident when parapost size 1.75 mm diameter was used in the study.
Currently, two types of posts are clinically used, cast posts and prefabricated posts. Although both types serve the same purpose, prefabricated posts have been found to have some advantages over cast posts. There is an economical advantage, in that most of the time they are made of stainless steel or non precious alloy. Also, a practical advantage is that treatment can be done in one visit. This approach is also more conservative because it does not require extensive removal of dentin and special tapering at the cervical level, Abou-Rass, M. (The prefabricated post selection and use in endodontic and restorative therapy. Clinical Dentistry, 4, Chap. 10B:1, (1985)); Miller, A.W. (Post and core systems, which one is best: The Journal of Prosthetic Dentistry, 48, (1), 27-38, (1982)).
In a clinical study involving 1,273 endodontically treated teeth, Sorrenson & Martinoff (1984) observed that the tapered cast post and core technique was most often used by dentists. Surprisingly, the clinical success of that technique (87.3%) was less than that of the endodontically treated teeth without intracoronal reinforcement (89.9%). In their investigation, the use of parallel sided, prefabricated or cast parapost was clinically successful (97.7%-100%). Success or failure in their study was based primarily upon whether or not the treated tooth had a non-restorable root fracture.
In their defense of the use of prefabricated parallel-sided posts over tapered cast posts, Sorenson & Martinoff explained that cast posts are commonly used because they follow the shape of conical or ribbon-shaped canals. Unfortunately, damage to the roots restored with this system was commonly observed. On the other hand, the use of prefabricated parallel-sided posts in conical or ribbon shaped canals requires a greater cement thickness; consequently, there was more dependance on the luting cement for retention.
The use of well adapted cast posts in their study resulted in some root fractures. Failure of prefabricated parallel-sided posts, on the other hand, was attributed to post dislodgement. Correspondingly, no damage to the root structure occurred. They concluded that resistance to root fracture was more important than post retention.
Factors that influence post retention are post design, the luting cement and the depth of posts.
Post design has been proved to be very effective on retention, Standlee, J.P., Caputo A.A., and Hanson E.C. (Retention of endodontic dowels: effect of cement, dowel length, diameter and design. The Journal of Prosthetic Dentistry, 39 (4), 401-405, (978)). Shillingburg, H.T., and Kessler, J.C. (Principles of restoration of endodontically treated tooth. Chicago, IL, Quintessence Pub. Co. Inc., 28, (1982)) concluded that surface configuration probably plays the single most important role in retention.
Posts can be classified according to their retention with surrounding dentin and gain their retention from dentinal threading. Passive posts do not engage dentin and are seated passively in their post channel. They gain their retention primarily from the surrounding cement medium, Musikant, B.L., and Deutsch A.S. (A new prefabricated post and core system. The Journal of Prosthetic Dentistry, 52 (5),631-634, (1984)).
Posts also may be classified according to their surface configuration. They may be parallel sided or tapered. Furthermore, they may be either serrated or smooth. Active type posts proved to exhibit the highest retentive values among all other post designs, Standlee et al. 1978; Cooley, I.T., Hampson E.L., and Lenman, M.L. (Retention of post crowns, and assessment of the relative efficiency of posts of different shapes and sizes. British Dental Journal, 124, 63-69 (1968)).
An active, parallel-sided, threaded post design provides the greatest retention. On the other hand, it generates stress on insertion and additional stress concentration under occlusal forces. Even though these forces may be distributed evenly along the entire length of the shank, they could lead to a root fracture. Longitudinal split in the long axis of the tooth has been observed in association with such a type of post design, Deutsch, A.S., Musikant, B.L., Cavallari, J., and Lepley, J.B. (Prefabricated dowels: a literature review. The Journal of Prosthetic Dentistry, 49 (4), 498-505, (1983)), Musikant & Duetsch 1984; Sorrenson, J.A., & Martinoff, J.T. (Clinically significant factors in dowel design. The Journal of Prosthetic Dentistry, 52 (1) , 28-35, (1984)) .
Active, tapered, threaded posts are less retentive than parallel threaded posts, but still provide higher retention than any passive post design (Musikant & Deutsch 1984). The problem with this type of post is that it exhibits a wedging effect and generates high shoulder stress concentration on the coronal area upon insertion and also under function, Deutsch et al. 1983; Standlee, J.P., Caputo, A.A. (The dentatus screw: comparative stress analysis with other endodontic dowel designs. Journal of Oral Rehabilitation, 9, 23-33, 1982. (1982)).
Durney, E.C. & Rosen, H. (Root fracture as a complication of post design and insertion: a laboratory study. Operative Dentistry, 2, 90-96, (1977)) reported that the torque required to insert tapered threaded post was almost 25% of the torque required to fracture a root. In the same study, parallel threaded post insertion was found to have no effect on root fracture. This finding was confirmed by Rivera, M. (The incidence of intraorganic fracture of brace roots. Master Thesis, McGill University, Montreal (1979)), who showed that tapered, self-threading posts tend to split teeth longitudinally during installation.
Passive posts are less retentive than active posts, because the luting cement plays a major role in their retention. The dislodgement of any passive posts occurs in response to shearing strain of the post-cement or dentin-cement interface (Standlee et al. 1978).
Passive parallel-sided posts have been shown to be far more retentive than tapered posts. This retention is increased if the post is serrated according to Cooley et al. (1968), Caputo and Standlee (1976), and Standlee et al. (1978). Musikant and Deutsch (1984) stated that parallel-sided passive posts distribute stresses evenly throughout the dentin, which then provides optimal protection against root fracture. The Parapost system is representative of this category, and it has become the most common type of prefabricated post used in restoration procedures.
Passive tapered posts are the least retentive posts especially when their surface is smooth. This design distributes stresses unevenly and acts like a wedge under occlusal loads, leading to increased stress concentration at the coronal part of the supporting dentin and may eventually lead to root fracture (Standlee et al. 1982; Musikant & Deutsch 1984).
Cooley et al. (1968) demonstrated that parallel-sided serrated posts cemented at a depth of 5.5 mm are more retentive than tapered posts at 8 mm depth. They also concluded that at least three factors influence retention strongly: the degree of post taper, the total involved surface area and lack of smoothness at the post surface.
In an in virto study dealing with endodontic posts, Chan, R.W. and Bryant, R.W. (Post core foundation for endodontically treated posterior teeth. The Journal of Prosthetic Dentistry, 48, 401 (1982)) used freshly extracted lower parallel-sided posts with amalgam or composite cores. Both groups were covered with full veneer crown. Their results showed that when a compressive load was applied to the veneered crowns, the cast posts and cores samples showed lower resistance and failed by post and core dislodgement or by root fracture. However, the prefabricated post samples showed higher resistance to dislodgement and fracture, and failed by amalgam or composite core fracture without causing the root to split.
Musikant and Deutsch (1984) generated the Flexi post design, which is an active, self-threading parallel post with blade-like threads characterized by a 7-mm split on the apical part of the post to compress upon itself during insertion, which then reduces insertion stresses and decreases chances of fracture. This design was meant to provide high retentive values while protecting remaining dentin from fracture.
Standlee, J.P., and Caputo, A.A. (The retentive and stress distributing characteristics of split, threaded endodontic dowels. Journal of Dental Research, 67 (Special Issue), Abstract 140, (1988)), in a photoelastic stress analysis study, demonstrated that the Flexi post exhibited minimal stresses during the insertion process, but under axial load, stresses were produced at each thread and at the coronal and apical levels. They explained that, as the shank of the post compressed, the parallel-shaped post actually becomes tapered and, consequently, produces a compressive stress on the walls of the chambers. They also demonstrated lower retentive values for the system in their study. Another study by Burns, A.A., Krause, W.R., Douglas, H.P., and Burns, D.R. (Stress distribution surrounding endodontics posts. The Journal of Prosthetic Dentistry, 64 (4), 412-418 (1990)) reported similar results, in that asymmetric patterns of stress distribution, stress concentration at each thread, greater shoulder stresses and substantial high stresses along the coronal surfaces were demonstrated for the Flexi post.
According to most of the reported investigation concerned with endodontic posts, the retentive values of all types of post designs can be listed in a decreasing order as follows: (active) parallel threaded, (active) tapered threaded, (passive) parallel serrated, (passive) parallel smooth, (passive) tapered serrated, (passive) tapered smooth.
Also, it has been documented that various types of posts produce different amounts of stress. In general in decreasing order they are: (active) tapered threaded, (active) parallels threaded, (passive) tapered and (passive) parallel.
From previous studies concerning post retention and reinforcement of tooth structure, it appears that no single post design is capable of producing high retention values, while at the same time providing maximum reinforcement for the endodontically treated tooth structure. Table 1 illustrates the shortcomings of known post design.
TABLE 1 __________________________________________________________________________ Classification of main post designs Post Design Advantage Disadvantage Mode of Failure Example __________________________________________________________________________ (Active) Highest retention High stress under Longitudinal split on Kurer Anchor Parallel-sided values load at: the channel on the long axis of Starr Dent. Mfg. threaded apex, the coronal the root Mfg. Co., level and around the Conshohoken, PA threads. More stress at short length. (Active) High retention Severe stress concen- Longitudinal split Dentatus screw Tapered, self- tration during on the long axis Dentatus, threading insertion and under of the root Stockholm, load. Sweden Wedge-like effect. Tend to split roots. (Passive) Distribution of Lower retention Dislodgement Parapost Parallel-sided stress evenly, values with cement Whaladent, Int., serrated optimal protection attached to NY for dentin. Provide the post for cement interlock (Passive) Less stress Uneven stress distri- Dislodgement with Custom cast post Tapered smooth concentration bution, wedge-like cement left in post & Kerr endo-post than active posts effect under load, channel, Kerr Mfg. Co. lowest retention root fracture values, stress con- centration at coronal level under compression load. __________________________________________________________________________
Luting cements provide posts with retention. The cement serves as an intermediate agent, which distributes the generated stresses evenly throughout the root. The failure of endoposts is related directly to inadequate retention. If this retention is not provided through dentinal threading, as in active post systems, then the luting cement should have a significant role in post retention.
Excessive axial loads transferred to a cemented post, according to Maniatopulos, C., Pilliar R.M., and Smith, D.C. (Evaluation of shear strength at the dentin-endodontic post interface. Journal of Prosthetic Dentistry, 59 (6) 662-669, (1988)) leads to failure at the cement-post interface in smooth posts and at the cement-dentin interface in threaded posts. According to Standlee et al. (1978), dislodgement of posts from their channels occurs because of a shearing stress directed at the post-cement interface or the dentin-cement interface, regardless of the direction of masticatory forces. Consequently, the bond strength of any luting cement to dentin or post surface should have a substantial effect on the prognosis of the clinical success of the final restoration. Any luting agent should have the ability to provide the associated post with adequate retention in its prepared channel
Standlee et al. (1978) also reported that the intensity, direction, frequency, and excessive forces on the endodontic post creates strains which could lead to root fracture, post fracture or post dislodgement. Another function for the luting cement is stress distribution. Perrel and Muroff (1972) indicated that the luting cement layer effectively redistributes the applied stress on the restoration to the surrounding dentin.
Leary J.M., Jensen, M.E., and Sheth, J.J. (Load transfer of posts and cores to roots through cements. Journal of Prosthetic Dentistry, 62, 298-302, (1989)) investigated the load transfer from cast posts and cores into the root through several luting cements. They demonstrated that the load was transferred extensively to the root, especially through zinc phosphate cement. This transfer was also observed with glass ionomer and Comspan cements but at lesser values.
Total stress in that study was measured before and after post cementation. Results showed that the total stress had increased in the body of the tooth after cementation, and that it was distributed evenly along the surrounding root area. These results lead to a conclusion that luting cements, such as zinc phosphate, which provided a good adaptation between the post and the surrounding dentin, are capable of distributing the applied load uniformly from the post through the body of the root. The cement is also capable of protecting the root from localized high stress concentrations. This finding agreed with the photoelastic study by Caputo, A.A., Standlee, J.P., and Collard, E.W. (The mechanics of load transfer by retentive pins. Journal of Prosthetic Dentistry, 29, 442-449, (1973)).
Zinc phosphate cement is the most common luting agent used to retain endodontic posts. It has been used since 1900 and has become the standard to which new luting cements are compared. Zinc phosphate cement powder is composed mainly of zinc oxide plus 10% magnesium oxide, which serves as a modifier for the cement. The powder components are prepared by firing to 1400.degree. C. prior to grinding into small powdery particles. The liquid in the cement is mainly 45-64% phosphoric acid plus 30-35% water, which is essential for the ionization. Aluminum phosphate is also added to the liquid as a buffer. As the powder and liquid are mixed, the phosphoric acid in the liquid dissolves the zinc oxide particles, while the aluminum ions help in forming a cohesive crystalline structure.
Zinc phosphate becomes a stiff cement after setting and resists elastic deformation. Its elasticity modulus is approximately 1.9.times.10.sup.6 psi. In 24 hours the compressive strength of zinc phosphate reaches 15,000 psi, with a tensile strength of 800 psi. Due to its lower tensile strength, it is quite brittle. The film thickness of the cement is approximately 20 .mu.m, Phillips, R.W. (Luting cements. Skinners science of dental materials 9th ed., pp 478-503, W.B. Sanders Co., Philadelphia, (1991)); Smith D.C. (Dental cements: current status and future prospects. Dental Clinics of North America, 6, (3), 763-792, (1983)).
One major problem with zinc phosphate cement is its solubility in water and oral fluids. Zinc phosphate is one of the most soluble of all luting cements, Phillips, (1991); Smith, (1983); Richer, W.A., and Veno, H. (Clinical evaluation of dental cement durability. Journal of Prosthetic Dentistry, 33 294, (1975)); Wilson, A.D., and Kent, B.E. (A new translucent cement for dentistry. The Glass Ionomer Cement. British Dental Journal, 132 (1972)). It is categorized as a temporary filling in restoring cavity preparations due to the erosion and disintegration of the material after long contact with moisture. The solubility of the cement in water ranges between 0.04 to 3.3% (standard 0.2%), and this solubility is increased dramatically up to 30% in organic acids, Phillips (1991), Craig, R.C. (Restorative Dental Materials 7th ed., St. Louis, The C.V. Mosby Co., (1985); Smith (1983)) .
Another major problem with zinc phosphate is its lack of adhesion to the tooth structure. Phillips (1991) explained that the manner in which zinc phosphate functions as a retainer for a restoration is through its flow into the irregularities of the surfaces of the restoration and surrounding tooth structure, thus bonding by mechanical interlock. Consequently, if either surface is highly polished, the mechanical property of the cement drops down precipitously. Comb, E.C. (Cements, adhesives and non-metallic filling materials. Notes in Dental Materials, 5th ed., NY, Levingstone Inc., (1986)); Worley, J.L., Hamm R.C., and yon Fraunhoffer, J.A. (Effects of cement on crown retention. Journal of Prosthectic Dentistry, 48, 289-291 (1982)) and Chan, R.W., Azzarbal P., and Kerber, P.E. (Bond strength of cements to crown bases. Journal of Endodontics, 10 (8), (1981)) reported, for example, poor adhesion to tooth structure and metal restoration by zinc phosphate cement when compared to other cement materials.
Table 2 shows comparisons of properties of zinc phosphate, glass ionomer, polycarboxylate and silico phosphate cement.
TABLE 2 __________________________________________________________________________ Comparison in properties of various luting cements decreasing order Film Compressive Tensile Modulus Solubility & Thickness Strength Strength of Elasticity Disintegration (.mu.m) (psi) (psi) (psi .times. 10.sup.6) (in vivo) Adhesion __________________________________________________________________________ Zinc phosphate Silico phosphate Silico phosphate Polycarboxylte Zinc phosphate Polycarboxylte (20) (21,000) (1100) (0.74) Polycarboxylte Zinc phosphate Polycarboxylte Glass ionomer Polycarboxylte Glass ionomer (21) (14,000-19,000) (900) (1.06) Glass ionomer Glass ionomer Glass ionomer Silico phosphate Silico phoxphate Silico phosphate (12,500) (900) (25) Silico phosphate Polycarboxylte Zinc phosphate Zinc phosphate Glass ionomer Zinc phosphate __________________________________________________________________________ Information and data are taken from Phillips (1991), Craig (1987) and Smith (1983).
Although luting cements exhibit a wide variation of properties from material to material, studies which involved the use of different cements as luting agents for post retention have demonstrated that there is no correlation between post retention and cement type, Hanson, E.C., and Caputo, A.A. (Cementing medium and retentive characteristics of dowels. Journal of Prosthetics Dentistry, 32, 551-557, (1974)); Standlee et al. 1978; Krupp, J.D., Caputo, A.A., Trabert K.C., and Standlee, J.P. (Dowel retention with glass ionomer cement. Journal of Prosthetic Dentistry, 44, 163, (1979)) .
Standlee et al. (1978) demonstrated that when zinc phosphate, polycarboxylate and epoxy resin were tested for the retention of a Parapost, the retentive values of these cements were not significantly different from each other. However, a significant increase in retention was obtained with zinc phosphate when a tapered smooth post was tested for retention. Epoxy resin was the least retentive cement in the study. It was pointed out in the same study that the luting cement always remained within the post channel whenever a tapered post was dislodged but under the same conditions the Parapost design allowed the cement to remain on the post surface.
Wood, W.W. (Retention of posts in teeth with non vital pulps. Journal of Prosthetic Dentistry, Dentistry, 49, (4) 504-506, (1983)) confirmed the findings of Standlee et al. in a study which involved testing tapered cast posts for retention, comparing zinc phosphate and composite resin as luting agents. Wood's results showed increased retention in favor of zinc phosphate cement over that of the composite resin. However, when retentive grooves were created in the post surface and adjacent grooves in the post channel, no difference was demonstrated in retention between zinc phosphate and composite.
Influenced by the reported adhesive properties of the polycarboxylate and the glass ionomer cement to dentin, Radake, W.A., and Veno H. (Clinical evaluation of dental cement durability. Journal Of Prosthetic Dentistry, 33, 294, (1988)) investigated the retention of the Parapost cemented with polycarboxylate, glass ionomer, zinc phosphate and composite resin. The results showed that the retentive values of glass ionomer and zinc phosphate cement were not significantly different from each other, but were higher than the retentive values of polycarboxylate and composite. Polycarboxylate cement was the next highest retention, while the composite resin showed the lowest retentive values in the study.
Chapman, K.W., Worley, J.L., and yon Frannhofer, J.A. (Retention of prefabricated posts by cements and resin. Journal of Prosthetic Dentistry, 54, 649-652, (1985)) also demonstrated that no changes in the retention values of Parapost were obtained by using zinc phosphate, polycarboxylate or glass ionomer cement. A difference in retention was shown when a strong posterior composite (P-10, 3M Company) was used as a luting cement. The retention values then dropped sharply. Chapman's conclusion was that the adhesion of the luting cement to dentin and post was more important than the shear strength of the cement itself.
Studies have shown that increasing the depth of the post provides an increased resistance to dislodgement. Cooley et al. (1968) demonstrated 2.23 times increased retention when the depth of the post was increased from 5.5 mm to 8 mm, while Standlee et al. (1978) showed a 1.5 times increase in retention.
Other studies showed different retention by changing the depth of the post, but all agreed that when the post depth in the post cavity preparation increases, the retention of that post also increases, Krupp et al. (1979); Johnson & Sakumara (1978); Ruemping, D.R., Lund, M.R., and Schnell, R.J. (Retention of dowels subjected to tensile and torsional forces. Journal of Prosthetic Dentistry, 41, 159-162, (1979)).
Hanson, E.C., and Caputo, A.A. (Cementing medium and retentive characteristics of dowels. Journal of Prosthetics Dentistry, 32, 551-557, (1974)) and Trabert, K.C., Caputo, A.A., and Hanson, E.C. (Effects of cement type and thickness on retention of serrated pins. Journal of Dental Research, 54, (2), 227-321, (1975)) pointed out that retention of any cemented post is affected by its adaptation to the post channel walls. A greater mismatch between the post and its channel results in decreased post retention. Chapman, K.W., Worley, J.R. and yon Frannhofer, J.A. (Effect of bonding agents on retention of posts. General Dentistry, 128-130, March-April (1985)) indicated that the luting cement's adhesion to dentin is more important than the shear strength of the cement.
Radake et al. (1988) tested several cements for the retention of an endo-post. They agreed that the bond strength of the cementing media plays an important role in the retention of the final restoration. Petters, M.C., Poort, H.W., Farah, J.W., and Craig, R.G. (Stress analysis of a tooth restored with a post and core. Journal of Dental Research, 62, 760-763, (1983)) also praised the post-cement bonding and reviewed it as the most important factor in achieving optimal mechanical behavior between the tooth and its restoration.
Chapman et al. (1985) reported that the use of resin as a luting cement is clinically acceptable. It can increase post retention, which then leads to a more conservative post cavity preparation as well as better preservation of the tooth structure. They added that the use of a bonding agent in conjunction with composite resin may add to the retention of the post.
Several authors relate the fracture of endodontically treated teeth to the corrosion of their posts in the root canal system. Rud J. and Omnel K. (Root Fracture Due to Corrosion. Journal of Dental Research, 78, 397-403, (1970)), in an in vivo study, inspected 468 extracted teeth that had failed due to root fractures. All teeth were restored with metal posts, cores and crowns. Their study showed that 71.8% of the fractures were related to pressure caused by corrosive agents. They indicated that corrosion most frequently occurred when the posts were made of stainless steel and contained tin. However, they recognized the capability of other metal components of causing corrosion. They also reported that if corrosion causes the root fracture, the fracture will always be vertical or oblique.
Derand T. (Corrosion of screw posts. Odontologic Revy, 22, 371-378, (1971)) studied the in vitro corrosion of gold-plated Dentalus screw posts, which are composed mainly of copper and zinc with some gold and silver. His results showed that corrosion had occurred 10% of the time, and corrosion products penetrated into the dentin.
In 1969 Angmar-Manson, B., Omnel K.A., and Rud J. (Root fracture due to corrosion-metallurgical aspects. Odontologist Revy, 20, 244-265 (1969)) analyzed the corrosion products from posts and their relation to the restorative material. Their analysis showed that post materials were made of either stainless steel; German silver alloy (copper, nickel and zinc); brass (copper and zinc); gold alloy; or alloy of copper, zinc and silver. The core build-ups of the post, according to the study, were made of amalgam, silver, gold or cast alloy (tin, zinc and silver). Nineteen teeth exhibiting corrosion in their posts were involved in the study. The chemical composition of the corrosion products were mainly tin and/or zinc, and, in some cases, iron, chromium and copper were present. They indicated that once the tissue fluids penetrates through the luting cement, or after it has dissolved, a prolonged electrolytic reaction between the dissimilar post and core material occurs. This leads to corrosion of tin and the presence of corrosion products, which are not capable of diffusion through the dentin. Considerable pressure then is exerted and fracture occurs, attracting more fluid and oxygen, which speed up the corrosion process and a compact corrosion layer is formed around the post.
An extensive investigation was carried out by Silness, J., Gustavsen F., and Hunsbeth, J. (Distribution of corrosion products in teeth restored with metal crowns retained by stainless steel posts. Acta Ondotologic Scandinay, 37, 317-321, (1979)) on fractured teeth with corroded endodontic posts, involving the use of energy-dispersive X-ray microanalysis, microradiography and electron microscopy. The study involved fractured teeth previously restored with stainless steel posts with amalgam core and cast gold crowns. They observed that the posts and their channel walls were covered with corroded material. Iron and chromium, plus other elements of calcium, phosphorous, zinc and tin were routinely present on the main fracture surfaces. The dentinal tubules near the post channel were completely obliterated with a dense material consisting of the same elements. Iron and chromium are the main components of stainless steel posts, while tin and zinc exist in the amalgam core.
One important observation in the study by Silness et al. (1979) was the presence of a radiolucent area adjacent to the corrosion products. This finding suggested that a demineralization process had occurred, making calcium and phosphorus available, which then served as electrolytes for the corrosive process.
Patterson, K.B. (Longitudinal fracture due to corrosion of an endodontic post. Journal of Canadian Dental Association, 37, 66-68, (1971)) observed that longitudinal root fracture occurred in teeth restored with stainless steel posts, amalgam cores and cast gold crown. Arvidson, K. and Wroblewski, R. (Migration of metallic ions from screw posts into dentin and surrounding tissues. Scandinavian Journal of Dental Research, 86, 200, (1978)), also observed corrosion products in the dentinal tubules around the post and also in the gingiva adjacent to the fractured tooth. The extracted teeth in their study had been restored with posts and had fractured 3-10 years after the treatment.
Goldman, M., De Vitre, R. and Pier M. (Effects of the dentin smeared layer on tensile strength of cemented posts. Journal of Prosthetic Dentistry, 52, (4), 485-488, (1984)) evaluated the effect of removing the smeared layer on the retention of a parapost cemented with zinc phosphate, polycarboxylate and Bis-GMA resin cement. They observed increased retention in all cement groups: 38% for polycarboxylate cement, 40% for zinc phosphate and 126% for resin cement group. The significant increase in resistance to dislodgement was twice as great with the Bis-GMA cement group as it was with zinc phosphate, and the Bis-GMA cement group was found to be 3 times stronger than the polycarboxylate cement group.
In another study, Goldman, M., De Vitre R., White R., and Nathanson, D. (Journal of Dental Research, 63, (12), 1003-1005,(1984)) demonstrated the capability of Bis-GMA-based unfilled resin to penetrate the dentinal tubules after the smeared layer was removed. This penetration was of sufficient magnitude to suggest a strong mechanical lock due to the high compressive strength of the material.
The Boston post system was designed and patented in 1987 and 1988 as a result of Goldman's group studies. The Boston post is a stainless steel or titanium, parallel-sided, threaded post, which fits passively into its post channel, after a matching twist drill creates a post cavity preparation slightly larger than the post's diameter. Before the post is cemented, the post canal is rinsed with 2.5 ml of 17% EDTA followed immediately by 2.5 ml of 5.25% NaOCL before it is dried with paper points and compressed air. The luting cement in the system is chemically cured Bis-GMA based unfilled resin with a low viscosity which is claimed to have substantial adhesive properties. The compressive strength value also is claimed to be around 30,000 psi. The luting cement bonds to dentin and flows into the serration of the post to provide a strong retention after it sets.
The retentive properties of the Boston post system was tested in comparison with Flexi post and parapost, using each post's recommended luting cement. The three post groups were cemented at a depth of 7 mm, and the results obtained showed higher separation loads for the Boston post group (mean of 80.7 lbs), followed by the Flexipost group (mean of 75.9 lbs) and the lowest mean value, parapost (35.9 lbs). The advantage of this post technique could be increased retention at a shorter post channel depth, which could reduce the chances of perforation, Nathanson, D., and Ashayeri, N. (Effects of a new technique CDA Journal, 27-31, November (1988)); Nathanson, D. (New restorative concepts for posts and cores. Journal of Clinical Dentistry , 1, (2) , 44-45, (1988)) .
Endodontic treatment involves cleaning, disinfecting and shaping of the root canal in preparation for a successful obturation. As a result of the mechanical instrumentation against the dentinal walls of the canal, a smeared layer is formed. Since 1975, authors have observed and described this layer in the root canal as thin amorphous debris which covers the instrumented surface, tending to block the lumen of the dentinal tubules, Moodnik, R.M., Dorn, S.O. Fledman, M.J., Levy, J., and Borden, B.G. (Efficacy of biomechanical instrumentation: a scanning electron microscopy study. Journal of Endodontics, Z, 261-266, (1976)); McComb, D., and Smith, D.C. (A preliminary scanning electron microscopic study of root canals after endodontic procedure. Journal of Endodontics, ! , 328-342, (1975)). Eirich, F.R. (The role of friction and abrasion in the drilling of teeth. In The Cutting edge: Interfacial dynamics of cutting and grinding, Ed. Pearlman, S., DHEW publication No. 76-670, pp 1-49, (1976)) reported that the smearing occurs when the hydroxylapatite in the dentinal tissues is "either plucked out or broken or swept along the resets, in the smeared out matrix" (pp 1-49).
Further investigation of the root canal smeared layer revealed that it contains organic and inorganic components, and that its thickness ranged between 1-5 .mu.m. Studies also have shown that this layer is not always firmly attached and not continuous with the dentinal surface, Gwinnett, A.J. (Smear layer: morphological considerations. Operative Dentistry, Supplement 3, 3-12, (1984)); Branstrom, M. (Dentin and pulp in restorative dentistry. London: Wolfe Medical Publications Ltd., (1982)); Moodnick et al. 1976; and McComb, D., Smith, D.C., and Bengrie, G.S. (The results of in vivo endodontic chemomechanical instrumentation-a scanning electron microscopic study. Journal of British Endodontic Society, 9, 11-18, (1976)). One scanning electron microscopic study by Mader, C.L., Baumgartener, J.C., and Peters, D.C. (A scanning electron microscopic investigation of the smeared layer on root canal walls. Journal of Endodontics, 10, 477-483, (1984)) reported the smeared material to be about 1-2 .mu.m in thickness, but was also found to be packed into the dentinal tubules for variable distances, sometimes up to 40 .mu.m.
Several studies have supported the need to remove the smeared layer because it can block the antimicrobial intra-canal medicaments from entrance to the dentinal tubules, which then may harbor necrotic tissue and bacteria. Another reason for detaching this layer is to facilitate the penetration and adaptation of dental filling material into the tubules and the canal walls, Yamada, R.S., Annabelle, A., Goldman, M., and Peck, A.L. (A scanning electron microscopy comparison of a high volume final flush with several irrigating solutions: part 3. Journal of Endodontics, 9, (4), 137-142, (1983)).
Pashley, D.H., Livingston, M.J., Reeder, O.W., and Horner, J. (Effects of the degree of tubular occlusion on the permeability of human dentin in vitro. Archives of Oral Biology, 23, 1127-1133, (1978)); Pashley, D.H., Michelisch, V., and Kehl, T. (Dentin permeability: effects of smear layer removal. Journal of Prosthetic Dentistry, 46, 532, (1981)); White, R.R., Goldman, M., and Lin, P.S. (The influence of the smeared layer upon dentinal tubule penetration by plastic filling materials. Journal of Endodontics, 10, (12), 558-562, (1984)); and White, R.R., Goldman, M., and Lin, P.S. (The influence of the smeared layer upon dentinal tubule penetration by endodontic filling materials. Journal of Endodontics, 13, (1987)) demonstrated that the smeared layer can be a barrier to fluid and other substances. It also can prevent the entry of the root canal filling material into the dentinal tubules, thereby decreasing the bond strength and adaption. If a dental cement is applied to the smeared layer that covers the dentinal surface, and if this cement is tested for its bond strength, the mode of failure will be either between the cement and the smeared layer, or between the components of the smeared layer itself, Pashley, D.H. (Smear layer physiological consideration. Operative Dentistry, Supplement 3, 13-29, 1984)).
Several solutions have been tested for removing of the smeared layer. Application of phosphoric acid at a concentration between 30-65% can achieve good clinical results. It also dissolves the peretubular dentin in a short period of time and enlarges the lumens of the dentinal tubules efficiently, Branstrom, M. and Nordenvail, K.J. (The effect of acid etching on enamel, dentin, and the inner surface of the resin restoration: a scanning electron microscopic investigation. Journal of Dental Research, 56, 917-923, (1977)). Gwinnett (1984) reported the successful effect of phosphoric acid on smeared layer but also indicted that "it appears to degrade the collagen matrix" (pp 9).
The smeared layer is calcific in nature. Therefore, it is reasonable to use a chelating agent such as citric acid, lactic acid or ethylene diamine tetracetic acid (EDTA). EDTA has been shown to be capable of decalcifying dentin at a depth of 20-30 .mu.m in 5 minutes. It has also been shown not to have irreversible pathological effect on the adjacent vital tissues, which makes it preferable over a more harsh material such as phosphoric acid, Goldman et al. (1981); Fehr & Nygaard-Ostby (1963).
However, EDTA alone is not capable of removing the organic debris which is also found within the components of the smeared layer. Sodium hypoclorite (NaOC1) is an appreciably better organic tissue solvent than a chelating agent, but it has little effect on the mineral components.
Yamada et al. (1983) studied the use of 10 cc of 17% EDTA buffered to pH =7.7, followed by 10 cc of 5.25% NaOC1 for removal of the smeared layer. Their results demonstrated a successful removal of the smeared layer over all of the root canal.
Curry, J.A., Bragotto, C., and Valdright, L. (The demineralizing efficiency of EDTA solution on dentin. I. Influence of pH Oral Surgery, Oral Medicine, Oral Pathology, 52, (4), 446-448, (1981)) reported that the maximal solubilization of calcium carbonate by EDTA can be accomplished at pH 7.3. Seidberg, B.H., and Schilder, P. (An evaluation of EDTA in endodontics. Oral Surgery, 37, 609 (1974)) showed that the action of EDTA stops once it reaches equilibrium with calcium ions in dentin, which means that EDTA has a self-limiting action.
Since the total volume of the root canal is fairly small, frequent refreshing of the EDTA within the canal was suggested to increase the exposed surface. This was proved to be more effective than one continuous application. When EDTA was applied in five increments for 3 minutes each, the results showed that the amount of dissolved mineral substances was twice that as when EDTA was applied in one increment for 15 minutes, Weinreb, M.H., and Meier, E. (The relative efficacy of EDTA, sulfuric acid and mechanical instrumentation in the enlargement of root canals. Oral Surgery, 19, 247-252 (1965)). Wu, J. (An in vitro investigation of pH changes of root environment after removal of the smear layer in endodontic treatment with calcium hydroxide. Masters thesis, UAB, Birmingham, AL (1988)) confirmed the successful removal of the smeared layer by irrigating the canal with 10 ml of 17% EDTA (pH =7.4) in 3 increments of 3 ml, 3 ml, and 4 ml, followed by 10 ml of 5.25% NaOCl.
Other methods of removing the smeared layer have shown favorable results, such as ultrasonic energized systems, which physically detach the debris from the root canal walls, followed by aspiration, Cunningham, W.T., Martin, H., Pellen, G.B., and Stoop, D.E. (A comparison of antimicrobial effectiveness of endosonic and hand instruments in R.C. therapy. Oral Surgery, 54, 238, (1982)). Cameron, J.A. (The use of ultrasound in the removal of the smear layer: a scanning electron microscopy study. Journal of Endodontics, 9, 289, (1983)) confirmed the efficacy of this system on removing the smeared layer.
Tidmarsh, B.G. (Acid-cleansed and resin-sealed root canals. Journal of Endodontics, 4, 117-121, (1978)), in an in vitro study, demonstrated the capability of a low viscosity unfilled Bis-GMA resin (used for fissure sealant) to penetrate the dentinal tubules at variable depth after the smeared layer had been removed.
Root canal plastic filling materials were introduced around 1984 for clinical use. Hydrophilic plastic (pHEMA) and silicon were applied as permanent root canal fillers after the smeared layer was removed. These fillings were tested to show their ability to penetrate the dentinal tubules (White et al. 1984 & 1987). White et al. (1987) also showed successful penetration into the tubules by other filling materials such as epoxy resin (AH26)+and zinc oxide and eugenol (Roth 801).
The number of dentinal tubules at 0.1-0.5 mm from the pulp ranges between 30,000-52,000/mm.sup.2. The tubular diameter at the same distance lies between 4-6.4 .mu.m (mean 2.5), while the tubular total surface area ranges from 9-42% (mean 22.1%). The diameter decreases gradually as the tubule approaches the dentin-cementum junction. It also decreases in a linear fashion within the root canal from the coronal dentin (44, 243/mm.sup.2) to the apical root dentin (8, 190/mm.sup.2), Garbeoglio, R., and Branstrom, M. (Scanning electron microscopy investigation of human dentinal tubules. Archives of Oral Biology, 21, 355-362, (1976)); Pashely (1984); and Carrington, P.J., Morse, D.R., Furst, M.L. and Siani, I.H. (A scanning electron microscopic evaluation of human dentinal tubules according to age and location. Journal of Endodontics, 10, (8), (1984)).
Factors that may influence the flow of any filling material into the tubules include: tubular radius, hydrostatic pressure gradient, tubular length and the viscosity of the filling material, Pashley (1984).
Bowen, R.L. (Dental filling material comprising vinyl silane treated fused silica and a binder consisting of the reaction product of bisphenol and glycidyl acrylate. U.S. Pat. No. 3,066,112, November, (1962)) introduced Bis-GMA resin, which has a higher viscosity than methylmethacryalate (MMA) and exhibits lower polymerization shrinkage. This resin is composed of Bis phenol A and glycidyl methacrylate (Bis-GMA), with the two organic compounds reacting to form an oligomer. The resin contains two double bonds capable of additional polymerization through the use of an initiator and an accelerator.
The polymerized Bis-GMA is highly cross-linked due to the presence of dysfunctional carbon bonds, Craig (1981); Lambrechts & Vanherle (1983).
To lower the viscosity of Bis-GMA, a diluent monomer is used, such as triethylene gylcoldimethacrylate (TEGDMA), a monomer which allows extensive cross-linking to occur between the chains, leading to increased resistance to solvent, but with a higher polymerization shrinkage.
Polymerization of dimethacrylate monomers is initiated by free radicals which are generated by chemical activation between an initiator (benzoyl peroxide) and an activator (tertiary amine).
Most dentinal bonding cements contain a reactive group linked to the methacrylate group by an aliphatic chain. The reactive group is believed to bond either to the calcium ions in dentin or to the inorganic part of the dentin. Phosphate is used as a reactive group in the dentinal bonding agent most of the time, Bowen (1962): Phillips (1991); and Craig (1985).
If Bis-GMA unfilled resin is capable of penetrating the dentinal tubules and, perhaps at the same time, can achieve some type of bonding action to the dentin, as Boston post system claims, then this system's major weakness may be the low bond strength expected between the resin cement and the metal.
Bonding between the metal surface and resin can be achieved through mechanical retention or chemical retention. Mechanical retention is provided through irregularities or undercuts on the metal surface, while chemical bonding can be obtained through an intermediate layer which is fused to the metal surface or by a chemical adhesive containing a coupling agent that attaches resin to metal. Mechanical beads, metal mesh, pitted metal, perforation of metal surface, electrolytic etching and chemical etching are several means for mechanical bonding, ADEPT Report (Pertinent information on cosmetic, adhesive and restorative dentistry. ADEPT Institute, Santa Rosa, Calif., 2, 2:25-39, Spring, (1991)); Livaditis, G.J., and Thompson, V.P. (Etched casting: an improved retentive mechanism for resin-bonded retainers. Journal of Prosthetic Dentistry, 47, 52-58, (1982)); McLaughlin, G. (One hundred second etch technique for etched metal bridges. Journal of Michigan Dental Association, 64, 347-349, (1982)); Alsobrook, C.S., Murray, S.G., and Yates, L.J. (Bond strengths of acid-etched bridge retainers. Journal of Pedodontics, 8, 387-392 (1984)); and Hudgins, J.L., Moon, P.C., and Knap, F.J. (Particle roughened resin-bonded retainers. Journal of Prosthetic Dentistry, 53, 471-475, (1985)).
A successful interfacial chemical bonding can be achieved through a pyrogenic silanization technique, known as silicoating. This technique was introduced to the U.S. in 1985. In general silicoating is a method in which a glass-like layer (SiOx--C) is applied on a toughened metal surface. This layer is capable of chemically bonding to resin through an intermediate silane layer. A silane bonding agent requires substrate end groups such as Si--OH to achieve a chemical bond. These groups are not available on the metal surface. The silicate surface tends to bond successfully to resin and remains stable even in a high moisture environment. In pure SiO.sub.2 glass, each Si atom is connected to four 0 atoms, and this rigid coupling gives the glass its property of brittleness.
In the silicoating technique, a carbon or hydroxyl group is incorporated into the molecule to provide a glass-like layer with decreased brittleness and increased elasticity, based on the concept of a SiOx--C layer. A modified surface is built up on a roughened metal surface (through sand blasting with 250 .mu.m aluminum oxide) to achieve a very thin silicoated layer. This is done through a flame pyrolitic deposition process, molecule by molecule, in the 10-20 angstroms size range, which is very small compared to the roughness on the sandblasted surface (1/100-1/100 times). It is essential for this layer to be as thin as possible because the change in SiO.sub.2 chemistry builds up strains in the broader region that increase as layer thickness increases.
The built-up SiOx--C layer is not sufficient for resin adhesion because the surface --OH group of SiO.sub.2 does not bond directly to the methacrylate group in resin. A bonding agent of silane type is therefore used to react with the --OH group. This occurs by splitting off the methanol (CH.sub.3 OH), forming an Si--O--Si bridge. The remaining organic surface (methacrylate) is polymerized with the resin, making a chemical bond between the resin and SiOx--C possible.
After the silanizing layer is applied, it is either bonded directly to the restoration resin or protected by a structure such as the Dentacolor opaquer, which is capable of chemical binding later with the resin. The use of unfilled resin coating on the silanization layer improves the resin wetting, giving the resin smaller contact angles where it meets with the coated surface.
The internal structure of the bonding layer (which is polymerized in organic frame work) becomes t similar to that of SiO.sub.2, leading to resistance to water sorption, Wilfred, B. (The adhesion of dental resins to metal surfaces).
Some properties of this system include: A resin-metal bond, free of marginal gaps, independent of metal type; A corrosion resistance adhesive bond by a very thin glass-like surface (0.1-1 micron) over the metal surface; and, A stable adhesiveness of glass surface to any metal.
Silicoater MD (Kulzer Inc., Irvine Calif.), a known silicoater system utilizes a primer Sililink (Kulzer Inc., Irvine CA) and an adhesive Siliseal (Kulzer Inc., Irvine CA). Sililink provides a metaloxide-dotted silicate layer, which is burned out on the sandblasted metal surface in the Silicoater MD (Kulzer Inc., Irvine CA) at a temperature of 600.degree. C. An elastic SiOx-layer, which is a thin interference layer with the metal surface, is built up in the firing process. After the surface cools, Siliseal is applied, which is the adhesion silane that link the SiOx-layer to the opaquer or the resin.
Several studies have compared the effectiveness of the Kulzer Silicoater system with other resin-metal bond techniques such as Ultra etch, Immersion etch, Lee primer, Gold link and Cover Up. The Silicoater system showed higher bond strength values between the metal and resin over that obtained in any other system, regardless of the resin used. Silicoated samples also showed stability of their properties under moistened conditions and after thermocycling up to 4000 times, for 24 hours.
Scanning electron microscope evaluation confirmed that the adhesion between the resin and the metal surface is gap free. Silicoated specimens under tensile bond strength also showed a cohesive failure within the resin most of the time, while other techniques showed adhesive failure at the metal-resin interface, Caeg, C., Leinfelder, K.F., Lacefiled, W.R., and Bell W. (Effectiveness of a method used in bonding resins to metal. Journal of Prosthetic Dentistry, 64, (1) , 37-41, (1990)); Naegeli, D.G., Duke, E.S., Schwartz, R., and Norling, B.K. (Adhesive bonding of composite resins to casting alloys. International Association for Dental Research, Abstract 798, Chicago, (1987)); Twesme, D.A., Lacefield, W.R., and O'Neal, S.J. (Effects of thermocycling, silicoating and etching on composite bonding to Cu, Au and Ni base alloys. International Association for Dental Research. Abstract 799, Chicago, (1987)); Kaiser, D., Malone W., Godoy, F., and Jones, T. (Three different retentive methods for the resin bonded retainer. International Association for Dental Research, Abstract 796, Chicago, (1987)); Belser, W., Bugnon, J., and Mayer, J.M. (Shear strength of resin bonded retainers using different retention/adhesion techniques. International Association for Dental Research, Abstract 137, Toronto, (1988)); and Norling, B.K., Murray, A.J., and Dal Santo, F.B. (Comparison of bond methods for veneering stainless steel crowns. International Association for Dental Research, Abstract 888, Toronto,(1988)).
By evaluating several studies and investigations related to posting systems and in relation to pulpless teeth, the following conclusions are reached.
Posts fail mainly because of the following: lack of post strength; lack of sufficient post retention within the root; and, root fracture.
Factors that induce root fracture include: dehydration of dentin by root canal treatment, heated endodontic pluggers, or heat from rotary instruments; reduction of the bulk of dentin by root canal treatment or post-cavity preparation; decreased elasticity of remaining tooth structure; residual stresses from lateral condensation of gutta percha, Felton, D.A., Webb, E.L., Kanoy, B.E., and Dugnoni, J. (Threaded endodontic dowels: effect of post design on incidence of root fracture, (1991)); stress from some post designs; hydraulic pressure during post cementation; endodontically treated immature teeth due to low mineralized dentin, less bulk of dentin and large canals, and larger and more dense dentinal tubules; corrosion of metal posts; and/or, Possible effect of H.sub.3 PO.sub.4 cement on dentin.
An evaluation of post designs and systems used in dentistry reveals that no post system is capable of providing high retentive properties and at the same time providing the weakened endodontically treated tooth structure with sufficient reinforcement without loading the remaining tooth structure with additional stresses. The same conclusion is also applied to the luting cements because no cement showed an overall success in all categories.
One problem that clinicians face is restoring endodontically treated teeth with short roots is the inability to increase post depth to provide additional retention. It is generally recommended that threaded active posts be used in this case. However, studies have shown high concentration stresses associated with loading short posts even when they are passive in design.
Another problem is the large canals in immature teeth which receive endodontic treatment. The amount of remaining mineralized dentin is considerably less than that in normal mature calcified teeth. In addition, the volume of the tubular dentin area is much higher and may be approximately 42% of the total surface area. Restoration of such teeth with an endodontic post or even leaving the canal without any reinforcement can lead to root fracture and extraction.
In other words, if the protection of the root is to be considered, the only available post design is the passive parallel post. Unfortunately, this design has low retention values because its retention relies on luting cements which are shown to be far from ideal.
On the other hand, if the retention of the post is a main concern, then the active threaded post design is best. Irreversible damage to the root structure have been associated with this type of post.
Therefore, there is a need for a post system which is capable of providing high retention values without compromising the protection of the root, while reinforcing the remaining root structure against possible fracture.